It is of advantage, especially in radio systems for transmitting transmitted signals by radio but also in line-connected transmission systems, if the transmitted signals can be received in various subfrequency bands of a receive frequency band. Individual subfrequency bands differ from one another with regard to their bandwidth and/or with regard to their frequency in the receive frequency band. By changing from a first subfrequency band to a second subfrequency band which has a greater bandwidth, transmitted signals with a greater transmission rate can be transmitted, for example. In a boundary case, the subfrequency band used is equal to the receive frequency band, i.e. the maximum available frequency bandwidth is utilized by the subfrequency band.
It is known to filter out a signal frequency band containing the transmitted signals by tuning the receive frequency band to a carrier frequency and by filtering the receive frequency band in a receiver. The signal frequency band is then demodulated in a demodulator so that a frequency baseband containing the transmitted signals is available at the output of the demodulator. The frequency baseband is processed further, for example, in that the information contained in it is digitized by means of an analog/digital converter and conditioned for its intended use as transmitted signals by subsequent fine filtering and/or further processing steps.
Surface acoustic wave filters (SAW filters) are known as filters for filtering a signal frequency band out of the receive frequency band as subfrequency band. However, SAW filters have the disadvantage of being relatively expensive to manufacture or procure.
To filter out subfrequency bands of different bandwidths in a radio-frequency section of a receiver, a plurality of SAW filters and/or SAW filters with different filter bandwidths are used. Each SAW filter or each filter bandwidth, respectively, corresponds to a bandwidth of one of the subfrequency bands which can be filtered out in the receiver. Because of the plurality of SAW filters or the plurality of filter bandwidths, such a receiver is relatively expensive. Furthermore, additional, relatively expensive switching elements, for example PIN diodes, are needed for switching to a different SAW filter or to a different filter bandwidth, respectively, when the subfrequency band is changed.
From U.S. Pat. No. 5,604,746, a method and receiver for receiving transmitted signals which can be transmitted in various subfrequency bands of a receive frequency band is known in which a first signal frequency band containing the transmitted signals is filtered out by adding a carrier frequency to the receive frequency band and by prefiltering the receive frequency band, in which a frequency baseband containing the transmitted signals is generated by demodulating the first signal frequency band by adding an intermediate frequency to the first signal frequency band, and in which at least one second signal frequency band containing the transmitted signals is filtered out of the frequency baseband by post-filtering, the carrier frequency and/or the intermediate frequency being matched to one or more filter parameters in the post-filtering in such a manner that the desired subfrequency band is available as a second frequency band.
It is the object of the present invention to specify a method and a receiver for receiving transmitted signals which can be transmitted in various subfrequency bands of a receive frequency band, the application or use of which requires little hardware costs.